Elements for incandescent devices



April 29,.1969 R. STEINITZ "ELEMENTS FCR INCANDESCENT DEVICES SheetFiled June 1, 1966 x CARBON ATOMS TANTALUM ATOMS INVENTOR.

ROBERT STEINITZ By a RNEX Fig. 3.

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April 29, 1969 v R. sTalNrrz 3,441,777

ELEMENTS FOR INGANDESCENT DEVICES Filed June 1, 19,66 0 1 Sheet 3 of2xlO' I, ATM. I 20 I TOC)( a. *1 o I 4 I I l l I I o 9 r v CARBON ATOMSTANTALUM ATOMS Fig. 2.

INVENTOR.

ROBERT STEINITZ Arra x United States Patent 3,441,777 ELEMENTS FORINCANDESCENT DEVICES Robert Steinitz, Harrison, N.Y., assignor toGeneral Telephone & Electronics Laboratories Incorporated, a corporationof Delaware Filed June 1, 1966, Ser. No. 554,422

Int. Cl. H01j 1/14, 19/06 US. Cl. 31'3 311 9 Claims ABSTRACT OF THEDISCLOSURE This invention relates to devices for operation at hightemperatures in a state of incandescence and, more particularly, toelements for use in an incandescent lamp.

Incandescent lamps generally comprise an electrically incandescibleelement, such as a filament, mounted between spaced support elements ina sealed enclosing en velope of a light-transmitting material, such asglass. The brightness and efiiciency of the lamp is determined primarilyby the properties of the electrically-incandescible element. The stateof incandescence is reached by heating the element to temperatures inexcess of 500 C.

While tungsten has been favored for use as the filament material, it isknown that improved brightness can be obtained by making the filament ofa more refractory material, such as tantalum carbide. Lamps utilizing atantalum carbide filament are capable of operation at higher levels oftotal radiation due to the fact that tantalum carbide has a highermelting point than tungsten and the total radiant output of anincandescent lamp is a function of the fourth power of the absolutetemperature at which the lamp filament is operated.

However, tantalum carbide has not generally replaced tungsten as anincandescent lamp filament since at high operating temperatures tantalumcarbide (TaC) tends to become unstable. Due to this instability, aninitially stoichiometric tantalum carbide filament decaburizes duringoperation. The carbon released in elemental form during decarburizationis found to deposit on the support elements and on the walls of theenclosing envelope thereby decreasing the ability of the envelope totransmit light. In addition, the decarburization of the filament canresult in the formation of TaC a subcarbide which is extremely brittle,has a lower melting point, and possesses an increased temperaturecoefficient of resistivity.

To limit this decarburization to acceptable levels, it has becomegenerally accepted practice to start with a stoichiometric TaC filamentand fill the envelope of the lamp with a protective atmospherecontaining a volatile compound of carbon such as methane, ethylene orcarbon tetrachloride. The gas dissociates within the envelope at the hotfilament so that as carbon atoms are ejected from the filament, othercarbon atoms bombard the filament surface and are retained. The amountof the volatile carbon 3,441,777 Patented Apr. 29, 1969 compound withinthe envelope has generally been at least that amount required tomaintain the filament essentially stoichiometric.

In addition, the amount of carbon present in the gaseous mixture may beincreased to a level such that a tantalum filament may be inserted inthe lamp and carburized by a controlled heating of the lamp. In thismethod of manufacture, the volatile carbonaceous gas is added inquantities of three to four times the amount calculated to completelyconvert the filament to the stoichiometric carbide.

This method of carburization produces a distortion of the filament,notably a sag, during carbnrization. This sag may be partiallyeliminated during lamp manufacture by selectively applying a colloidalsuspension of graphite to a tantalum filament and employing acarburizing gas in the manner described in US. Patent 3,113,893 to H. B.Sloan issued Dec. 10, 1963.

While the above methods have enabled tantalum carbide filaments to bemade which are initially stoichiometric with a composition TaC, the useof protective carbon-containing environments in practice has been foundto inhibit but not eliminate the decarburization of TaC. Decarburizationof the stoichiometric TaC is still found to occur, with the releasedcarbon depositing on the cooler surfaces of the envelope.

Also, stoichiometric TaC filaments exhibit a temperature coefiicient ofresistivity 0c of approximately 25 x 10-*/ C. The temperaturecoefficient of resistivity is determined from the following expressionwherein R is the electrical resistance at temperature T and R is theresistance at temperature T Thus, the coefficient a is indicative of thechange of resistance with temperature. Since it is advantageous toeliminate current surges during the operation of a lamp, the coetficientshould be minimized for lamp filaments.

The creep resistance of a lamp filament is an important parameter in theconstruction of a lamp since filaments by their nature are operated attemperatures of the order of 3500 K. The creep rate for a material isexpressed as Al/ (l At) where Al is the change in length from theinitial length 1 during a period A! for a given constant applied loadand temperature. As known, the creep rate of a material increases withincreasing temperature so that at high lamp temperatures the tendencyfor lamp filaments or supports therefor to sag and deform increases. Thecreep rate for fully carburized stoichiometric tantalum carbide at aload of 10,000 p.s.i. and a temperature of 1960 C. has been observed tobe within the range of 2 to 5% per hour. This creep rate may result inan undesirable sagging and deformation of the tantalum carbide filamentand/or support elements when the lamp is placed in operation.

Accordingly, an object of the present invention is a method of making animproved incandescent device containing a tantalum carbide element.

Another object is to provide an incandescent lamp having a tantalumcarbide incandescent element in which the temperature coefiicient ofresistivity is minimized.

A further object is to provide a device for operation at incandescenttemperatures having a tantalum carbide element which exhibits reducedsag and deformation.

Still another object is to provide a lamp containing a tantalum carbideincandescent element wherein the amount of elemental carbon lost duringoperation is substantially reduced.

Yet another object is to provide an improved tantalum carbide supportelement for use in an incandescent lamp.

In accordance with the present invention, a device to be operated in astate of incandescence is initially provided with a tantalum carbideelement which deviates from stoichiometry in a prescribed manner. In theformation of the device, the element is made of a nonstoichio metriccarbide having the general compositional formula (Ta,A)C wherein x isless than 1.0 and A is a metal additive not in excess of 20 weightpercent of tantalum. This additive A consists of at least one metalselected from the group consisting of zirconium, hafnium, niobium andtitanium. The quantity x, which indicates the ratio of carbon atoms tothe total number of metal atoms, is selected to be within the range of0.98 and 0.78.

By initially incorporating a filament of nonstoichiometric compositionin an incandescent lamp, the decarburization of the filament in vacuumor in a protective or inert atmosphere is substantially reduced and theelectrical and mechanical properties thereof are maintainedsubstantially constant. This result is obtained because the evaporationrate of carbon is found to decrease rapidly with a deviation fromstoichiometry. The vapor pressure at 2500 C. for carbon of a tantalumcarbide filament wherein x is 0.98, has been found to be about 15% ofthe vapor pressure for a stoichiometric carbide filament. As a result ofthis relatively low vapor pressure, fewer carbon atoms are availablewithin the lamp and the initial blackening of the lamp envelope byelemental carbon depositing thereon is substantially reduced.

Further, it has been found that initially employing a tantalum carbidefilament that deviates from stoichiometry in the prescribed mannerprovides a lamp which exhibits a reduced change in resistance during theperiod between energization and full operating temperature. This resultis due to the low temperature coefiicient of resistivity of low carbontantalum carbide filaments wherein the aforedefined x resides within therange of 0.98 to 0.78. The temperature coefficient for a filamentwherein x is 0.92 is found to be one-third of that for a stoichiometricfilament. An even more significant change is observed as x decreases to0.82 wherein the coefiicient is about one-fiftieth that of thestoichiometric filament.

The initial placement of a tantalum carbide element deviating fromstoichiometry in an incandescent lamp has also resulted in animprovement in the mechanical properties of the element. While tantalumcarbide is known to be relatively brittle at room temperatures, thematerial becomes quite ductile at temperatures above 17 C. The creeprate for fully carburized or stoichiometric tantalum carbide elements isfound to be substantially greater than that of tantalum elements havingthe aforementioned deviation from stoichiometry. As a result, tantalumcarbide filaments and supporting elements therefor deviating fromstoichiometry in the prescribed manner exhibit improved dimensionalstability during the operation of the incorporating device.

Further features and advantages of the invention will become morereadily apparent from the following detailed description of a specificembodiment of the invention when taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a representative incandescent lamp constructed in accordancewith the invention;

FIG. 2 is a curve showing the variation in carbon vapor pressure of atantalum carbide filament as a function of the deviation fromstoichiometry x; and

FIG. 3 is a curve showing the variation of the temperature coefficientof resistivity a as a function of the deviation from stoichiometry x.

Referring now to FIG. 1, a tantalum carbide incandescent lamp is showncomprising a vitreous light transmissive envelope 11 and a baseconnection 12. A coiled filament 13 of tantalum carbide is supportedwithin the envelope by support elements 14 and 15 which may also beformed of tantalum carbide but have a large diameter (i.e., lowresistance) so that they do not become incandescent.

In the manufacture of an incandescent lamp, the support elements 14 and15 are embedded in a glass plate or disc. The support elements generallyare formed as rods and may be provided with a sleeve formed of amaterial, such as Kovar, having a temperature coefficient of expansionclosely matching that of the glass. The glass plate is then softened byheating to form a stemlike structure containing the support elements.

The filament 13 is affixed by welding or the like to the ends of thesupport rods as shown in FIG. 1. The stem is then mated with an envelope11. In the mating of the stem with the envelope, a passage is providedfor the evacuation of the envelope atmosphere and for any back-fill thatmay be desired. This passage may be formed in the stem or the envelope.

The envelope is normally evacuated and back-filled with a carboncontaining protective atmosphere to a pressure within the approximaterange of 400 millimeters of mercury to one atmosphere. The gas maycontain a hydrocarbon, such as methane, or ethane, or a cyanogen oranother carbon-containing compound in order to provide the carbon atomsnecessary to prevent the decarburization of the filament. However, theprotective atmosphere may be omitted if desired and an inert atmospheresubstituted therefor or the envelope may be maintained in vacuum.

The evacuation passage is sealed after back-filling, if desired, bylocalized heating and the application of pressure. The exposed portionof the stem is then provided with a conventional socket with the supportelements being electrically connected to different portions thereof.

The filament 13 may be a straight wire, a coil or a coiled-coil, thatis, a doubly coiled filament, and is supported by and electricallyconnected between support elements 14 and 15. The filament is formed ofa nonstoichiometric tantalum carbide compound having the general formula(Ta,A)C wherein Ta is tantalum, C is carbon, and A is at least one metalselected from the group consisting of zirconium, hafnium, niobium andtitanium. The amount of additive A is chosen to not exceed 20 weightpercent of the tantalum. The parameter x indicates the deviation fromstoichiometry of the compound and is the ratio of carbon atoms to thesum of metal atoms and is within the range of 0.98 to 0.78.

In accordance with the invention, a filament having the abovecomposition is initially placed within the lamp during its manufacture.This is in contradistinction to the well known method of making anincandescent lamp with an initially stoichiometric filament and havingit decarburize during use. One advantage obtained by initially employinga nonstoichiometric tantalum carbide filament is that thedecarburization rate is greatly reduced with the result thatsubstantially no elemental carbon is deposited on the envelope duringnormal use and the properties, such as the temperature coefficient ofresistivity of the filament are maintained substantially constant duringoperation.

This advantage is shown graphically in FIG. 2 wherein the carbon vaporpressure for several tantalum carbide filaments having different valuesof x from 1.0 to 0.72 is plotted as curve 17. The data of curve 17 werecalculated for a temperature of 2500" C. It shall be noted that thecarbon vapor pressure decreases from in excess of 36x10 atm. for aninitially stoichiometric filament having x essentially equal to 1, toless than 7 X 10* atm. for a filament having x initially equal to 0.98.A further improvement is apparent for filaments having an initialincreased deviation from stoichiometry with the vapor pressure becomingas low as 1 10- atm. at x equal to 0.80. The evaporation rate of carbonfrom the filament is proportional to its vapor pressure.

By employing a tantalum carbide filament which deviates fromstoichiometry in the prescribed manner, the decarburization process ofthe filament is greatly reduced as shown by the much reduced vaporpressures of FIG. 2. Consequently, the amount of elemental carbon givenoff by the filament and the deposition thereof on the envelope or thesupporting rods 14, 15 is correspondingly decreased.

The use of a tantalum carbide filament having the initial prescribedlow-carbon composition in the manufacture of an incandescent lampprovides a filament having a reduced temperature coefficient ofresistivity. Since in normal use an incandescent lamp is used with aconstant voltage source, it is desirable to maintain the resistance ofthe lamp constant during the interval from initial energization to theattainment of the normal high operating temperature to prevent currentsurges.

A lamp constructed in accordance with the invention is found to exhibita substantially constant low temperature coeflicient of resistivity.This result is clearly shown in FIG. 3 wherein the coefficients fortantalum carbide filaments having different deviations fromstoichiometry were measured at about +155 C. and again at thetemperature of liquid nitrogen -l96 C. The temperature coefficient a wasfound to be the same for both high and low temperatures.

The temperature coefficient, as shown in FIG. 3, was found to decreaserapidly with increasing deviations from stoichiometry and found to reacha minimum value of about 0.5 l per C. at x equal to 0.82. It shall benoted that for x equal to 0.99 the average temperature coefiicient wasfound to be about 25 10-' per C. and that at x equal to 0.98 thecoefiicient was found to be reduced to bout 20 10* per C.

The temperature coeflicient of FIG. 3 was found to increase again as xdecreased below about 0.78. For tantalum carbide compositions havinglower carbon content, i.e., increased deviation from stoichiometry, thematerial enters a two-phase region consisting of TaC and Ta C. Thischange in the crystal structure results in a material wherein each phasehas its own properties, including a lower melting point and a hightemperature coefiicient of resistivity, and the distribution of phasescannot be readily controlled or predicted. Therefore, filaments formedof TaC wherein x is less than 0.78 are generally not well suited for usein incandescent lamps.

In practice, it has been found desirable to insure a single phasefilament and this may be provided b establishing a compositional limitfor x of 0.80. While suitable filaments may be formed at x as low as0.78, the use of the higher value is prefer-red to prevent the filamentfrom decomposing to an additional phase in a few localized regions. Inaddition, the carbon vapor pressure shown in FIG. 2 becomes quite low,less than 1 l0 atm., at x equal to 0.80 and remains relatively constantfor lower values of x.

The preferred compositional range for the tantalum carbide filamentsemployed in the present invention has been found to be where x may varybetween 0.95 and 0.80. The upper value 0.95 is the point whereat thetemperature coeflicient x of a filament is one-half that f thestoichiometric filament.

While the above discussion has referred to lamps initially containing atantalum carbide filament having the defined deviation fromstoichiometry, it will become apparent from the following discussionthat the supporting elements 14 and 15 of FIG. 1 may likewise be formedof this material.

As known, tantalum carbide is a brittle material at room temperatures.However, at relatively high temperatures above about 1700 C., thematerial becomes quite ductile and shows elongation in short timetensile tests of about 40% at 2200 C. and about to at temperatures above1900 C. The elongation rate under constant load (creep rate) was foundto be decreased substantially for tantalum carbide having the prescribeddeviation from stoichiometry. This property enables filaments andsupports formed of this material to exhibit improved dimensionalstability at high temperatures characterizing the state ofincandescence. In tests performed at a temperature of 1960 C. and aconstant load of 10,000 p.s.i., the creep rate for fully carburized TaCwas found to reside within the 2 to 5% per hour range. However, forsamples having the composition TaC and TaC the creep rates were found tobe 0.3% per hour and 0.03% per hour respectively. Further, the hardnessof the samples, i.e., the microhardness of the individual tantalumcarbide grains, was found to increase from 1200 kgn/mm. for TaC to 24-00kg./mm. for oas- The improved properties exhibited by filaments andsupport elements in a lamp apply to elements, both filamentary andsupporting, having the prescribed composition which are incorporated inother types of devices to be operated in a state of incandescence. Forexample, the elements may be used in infrared sources, rocket nozzles,or other devices for operation at the high temperatures characterizingincandescence.

Although many methods can be employed to prepare tantalum carbidefilaments and support elements, it has been found advantageous topreform the elements from tantalum sheet or wire and then carburize themto the desired non-st-oichiometric carbon content by heating incrucibles of tantalum carbide having the desired carbon to tantalumratio. In the fabrication of the tested samples, the formed tantalumcarbide elements were heated in this manner for 3 to 5 hours attemperatures of about 3000 C.

While the above description has referred to specific embodiments of theinvention, it is apparent that many modifications and variations may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:

1. A filament for later inclusion in an incandescent lamp consistingessentially of a n-on-stoichiometric carbide having the general formula('Ia,A)C wherein x is within the range of 0.98 to 0.78, Ta is tantalum,C is carbon and A is at least one metal selected from the group ofmetals consisting of zirconium, hafnium, niobium and titanium, in anamount not in excess of 20 weight percent of tantalum.

2. Apparatus in accordance with claim 1 in which x resides within therange of 0.95 to 0.80.

3. Apparatus in accordance with claim 1 in which x is approximately0.82.

4. A filament support for later inclusion in an incandescent lampconsisting essentially of a non-stoichiometric carbide having thegeneral formula (Ta,A)C wherein x is within the range of 0.98 to 0.78,Ta is tantalum, C is carbon and A is at least one metal selected fromthe group of metals consisting of zirconium, hafnium, niobium andtitanium, in an amount not in excess of 20 weight percent of tantalum.

5. Apparatus in accordance with claim 4 in which x resides within therange of 0.95 to 0.80.

6. Apparatus in accordance with claim 4 in which x is approximately0.82.

7. In the manufacture of an incandescent lamp of the type having afilamentary element mounted'on at least one support element, the stepwhich comprises initially forming at least one of said elements thereinof nonstoichiometric tantalum carbide having the general formula (Ta,A)Cwherein x is within the range of 0.98 to 0.78, Ta is tantalum, C iscarbon and A is an additive consisting of at least one metal selectedfrom the group consisting of zirconium, hafnium, niobium and titanium inan amount not in excess of 20 weight percent of tantalum.

8. The method of claim 7 in which x resides Within the range of 0.95 to0.80.

7 8 9. The method of claim 7 in which x is approximately 3,022,437 2/1962 Cooper 313-218 0.82. 3,022,438 2/1962 Cooper 313218 X 3,022,439 2/1962 Cooper 313-218 X References Clted 3,237,284 3/11966 Bird 313218UNITED STATES PATENTS 5 3,277,330 10/1966 Cooper 313218 X 1,854,9704/1932 Agte 313311 OHN HUCKERT P E 2,030,695 2/1936 Erber 3132-18 Jxamfner 2,072,788 9 7 Andrews 313 2 X J. R. SHEW-MAKE'R, AsszstantExannner. 2,596,469 5/ 1952 Cooper 313218 I 2,928,977 3/1960 Roth 3 13 311 X 10 3,022,436 2/1962 Cooper 3131-218 3513218; 316-1

